The present invention relates generally to crystal oscillators and more specifically to a temperature compensated crystal oscillator having a compensating circuit that stabilizes the output frequency of the crystal oscillator over a desired temperature range.
Crystal oscillators are commonly used for a number of applications that require a stable output frequency. The output frequency, however, varies as a function of the ambient temperature of the oscillator. FIG. 1 shows a graphical representation of the frequency of a typical uncompensated AT cut quartz crystal versus the ambient temperature. As shown the curve 6 has a generally cubic curve shape that can be characterized by three temperature regions. The curve in the cold temperature region (xe2x88x9235xc2x0 C. to approximately +10xc2x0 C.) has a linear portion having a positive slope and a nonlinear portion wherein the slope of the curve changes polarity. The curve in the middle temperature region (+10xc2x0 C. to +50xc2x0 C.) has a linear portion having a negative slope. The curve in the hot temperature region (+50xc2x0 C. to +90xc2x0 C.) has a linear portion having a positive slope and a nonlinear portion wherein the slope of the curve changes polarity. The point of inflection 8 is in the middle temperature region at approximately +28xc2x0 C.
A number of techniques to compensate for this frequency variation of the crystal includes the use of analog circuitry. One such analog compensation technique uses a resistor/thermistor network. For temperature range applications that extend into the non-linear portions of the AT cut crystal curve, at least three thermistors are necessary to compensate for each temperature region. Negative temperature coefficient thermistors are put into a network with a number of fixed resistors. The network is then supplied with a stable, fixed voltage source. By selecting the proper thermistors (for nominal value and temperature slope) and the value of the fixed resistors in the network, it is possible to match a variety of xe2x80x9cATxe2x80x9d cut crystals and cancel the frequency vs. temperature drift over a wide temperature range. Stabilities of better than 0.5 ppm can be achieved with this method over a temperature range of xe2x88x9240xc2x0 C. to +85xc2x0 C.
While this technique is well suited to some applications, there are some disadvantages which limit wider usage. First, a wide range of precision, tight tolerance resistors (usually 1% or better) must be stocked. Second, a set of resistor values unique to each oscillator must be selected and manually installed. Third, the calculations and measurements necessary to select these components result in a time consuming process of iteratively testing, changing components, and re-testing the oscillators until they have been xe2x80x9cmassagedxe2x80x9d to meet the specifications. Fourth, the thermistors must also be selected so that the thermistor slopes and ratios match the crystal being used. Fifth, interactions between the thermistors in the combined network limit the precision of the compensation that can be achieved. Sixth, because of the simple voltage divider action of the network, the output voltage has a limited dynamic range making operation at low voltages impractical.
Some attempts at automating the xe2x80x9cmassagingxe2x80x9d process by trimming the resistors and matching the crystal have been successful for specific applications with moderate stabilities, however full automation has proven to be very difficult. Further, it is impossible to reverse the trimming process in order to decrease the resistance of the trimmed resistor. Resistors which are trimmed are typically screen printed onto the circuit substrate, and therefore cannot be simply replaced.
One method for tuning a crystal oscillator is shown in U.S. Pat. No. 5,473,289. A single linear temperature sensor is used as implemented by one or more diodes. This combination produces a straight line function of voltage vs. temperature which is then applied to a plurality of voltage function generator circuits that generate a series of straight line segments of varying slopes and intercepts. A switching circuit then controls which segment is active at a given temperature, in effect summing all of the segments over the operating temperature range to give an approximation of the crystal curve made up of a series of straight lines. A drawback with this approach is that the compensation voltage does not generate a smooth match to the cubic crystal curve, but rather employs discrete, distinct segments with crossover points to approximate the cubic crystal curve.
Another analog method uses thermistor/capacitor networks in a similar manner as the resistor/thermistor networks by adjusting the effective reactance of one or more fixed capacitors as the temperature varies. This method is very cost effective and has been produced for consumer applications requiring moderate stabilities of +/xe2x88x922.5 ppm over a tighter temperature range. For applications that require operation in a wider temperature range, tighter matching of the components is required which becomes increasingly difficult due to the inability to match the crystal slopes and limitations in component values, tolerances and stabilities.
Another analog method uses multipliers that multiply a voltage which is linearly proportional to temperature which then generates a square and cubic term. These signals are then scaled appropriately and added together to produce a third order polynomial which matches the crystal curve to be compensated. This process still requires a set of resistors to be selected or trimmed and therefore, requires subsequent corrections which requires physical replacement or modification of one or more resistors.
A digital compensation technique includes the use of look-up tables. The frequency differential crystal curve between a selected temperature range is stored in a look-up table. The binary data stored at each memory location of the look-up table contains a compensation value that corresponds to each temperature increment. The output of a linear temperature sensor is digitized over the operating temperature range by an analog-to-digital (A/D) converter. The output from the A/D converter addresses the look-up table stored in nonvolatile ROM. The selected binary compensation value that corresponds to the ambient temperature of the oscillator is converted to a voltage by a digital-to-analog (D/A) converter which is used to tune the frequency of the crystal oscillator.
The ultimate stability obtainable by this approach is determined by the resolution of the A/D and D/A converters. Stabilities better than the hysteresis and repeatability of an AT cut crystal (about 0.05 ppm) are achievable over some temperature ranges with the proper system design.
All digital compensation systems, however, exhibit some degree of quantization noise, caused by the discrete steps of the conversion process. This is seen as a discrete jump in the output frequency as the compensation is updated. This effect can be minimized by increasing the resolution of the converters and filtering of the output, but it is very difficult to reduce it below the tolerance threshold of some systems. Spurious noise caused by feedthrough and coupling of digital switching components can also be a severe problem.
Another digital compensation technique used a microcomputer. This greatly reduced the amount of non-volatile programmable memory that is needed since interpolation or curve-fitting routines required much less stored data. Some success at Application Specific Integrated Circuit (ASIC) implementation has been achieved, but various issues have prevented these oscillators from being widely used.
The latest approaches for microcomputer compensation have used crystal self-temperature sensing techniques for best accuracy and repeatability. This method operates the crystal on both the fundamental and third overtone modes simultaneously. This is usually done with an SC cut crystal, but it is also possible with AT cut crystals. The apparent angle shift between the modes produces a signal which is very accurately proportional to temperature when the difference between the third overtone frequency and the fundamental multiplied by three (3) is compared. Since the frequency of these oscillators cannot be adjusted or tuned without affecting the calibration of the thermometer signal, external means for generating the stable output frequency must be employed.
This type of microcomputer controlled TCXO utilizing crystal self-temperature sensing has achieved the best stability of any compensated oscillator. Overall stabilities of better than 0.05 ppm over xe2x88x9255xc2x0 C. to +85xc2x0 C. have been reported. The complexities of these oscillators, however, make them relatively expensive, and they still suffer from some of the noise problems which are inherent in digital compensation systems.
Accordingly, it is an object of the present invention to provide a cost effective crystal oscillator that has a highly stable output frequency over a desired temperature range.
It is another object to provide a crystal oscillator wherein the tuning of the output frequency may be easily adjusted.
It is a further object to provide temperature compensated crystal oscillator that eliminates the need to trim or select the proper component value.
The above and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.
In one aspect of the present invention, an electrical device to compensate for crystal oscillator frequency shifts occurring over a temperature range includes a voltage divider for generating a temperature variable, compensation voltage at an output. The output of the voltage divider is to be electrically coupled to the oscillator so that the compensation voltage compensates for the crystal oscillator frequency shifts otherwise occurring over the temperature range. A voltage source is to be coupled to an input of the voltage divider for inputting a generally fixed voltage during normal crystal oscillator operation, and providing for multiple and repeatable adjustments to the fixed voltage before beginning the normal crystal oscillator operation.
In another aspect of the present invention, a method for tuning a crystal oscillator that has a temperature compensating circuit and a voltage source for inputting a tuning voltage to the temperature compensating circuit includes the steps of a) providing a tuning voltage from the voltage source to the temperature compensating circuit; b) determining a frequency variation range of the crystal oscillator over a temperature range; c) comparing the frequency variation range to a desired frequency operating range; d) setting the voltage source to permanently generate the voltage if the frequency variation range is equal to or less than the desired frequency operating range; and e) adjusting the voltage source to generate a different tuning voltage if the variation range is greater than the desired frequency operating range and repeating steps a-e with the different tuning voltage.
In a further aspect of the present invention, a temperature compensated crystal oscillator includes a voltage tunable crystal oscillator having a crystal with a resonant frequency that varies over a temperature range. A voltage divider generates a temperature variable, compensation voltage at an output. The output of the voltage divider is to be electrically coupled to the oscillator so that the compensation voltage compensates for the crystal oscillator frequency shifts otherwise occurring over the temperature range. A voltage source is to be coupled to an input of the voltage divider for inputting a generally fixed voltage during normal crystal oscillator operation, and providing for multiple and repeatable adjustments to the fixed voltage before beginning the normal crystal oscillator operation.